This API is not public so doesn’t have documentation, may not work as you expected and can be changed at any time.
You have to use api/images api instead of api/files
http://vzhuk.fibery.io/api/images/c863dbdd-2bb1-462e-81a6-8ba2ad6a3b18?width=320&height=0&dpr=2
Currently this endpoint accepts only images which is less or qual to 21 megapixels in resolution (width*hight) due to perf reasons.